The invention relates to improvements in hydromechanical drives, and more particularly to improvements in hydromechanical drives of the type wherein a prime mover can transmit torque to one or more driven components (e.g., to the wheels of a motor vehicle) by way of a hydromechanical clutch, a hydromechanical transmission or an analogous torque transmitting unit.
German Auslegeschrift No. 19 45 440 of Heyl discloses a hydrostatic axial piston transmission with internal torque division. The arrangement is such that the output element can be selectively coupled to the input element or to a rotary housing through the medium of an intermediate gearing. The transmission further comprises a braking system which can be used to vary the RPM of the output element or the RPM of the intermediate gearing. A drawback of the transmission of Heyl is that it must rely on the action of the braking system which is subject to extensive wear. Moreover, the intermediate gearing can be set only to establish or to interrupt a driving connection between the input element and the output element; changes in speed must be effected exclusively by the braking system.
German Utility Model No. 74 25 346 of Moeller & Neumann discloses a differential which employs a hydrostatic motor, particularly a radial piston machine, with an inlet and an outlet for the hydraulic fluid. The motor has a rotor as well as a rotary housing; one of these rotary parts is connected with the input element and the other rotary part is connected with the output element of the differential. Each of the rotary parts can receive hydraulic fluid at a variable rate. The source of pressurized fluid is a hydraulic pump which can supply fluid at a variable rate and determines the RPM of the hydrostatic motor when the differential is called upon to superimpose a second RPM upon a basic RPM. The differential further employs a turnable joint which serves to supply pressurized fluid as well as to provide a path for return flow of the fluid. The means for supplying and returning hydraulic fluid is an open system which is driven by an axial piston pump and is equipped with a pressure relief valve. There is a vessel which supplies fluid to the pump and a vessel for collection of returning fluid. A drawback of the just described differential is that the hydraulic system merely constitutes an auxiliary unit and that it is not possible to accurately regulate the entire RPM range of the differential.
German Offenlegungsschrift No. 36 19 671 of Hiramatsu discloses a hydrodynamic transmission and a control circuit which receives signals denoting the RPM of the input and output elements and generates signals which are used to control the operation of a mechanical clutch which is connected in parallel with the hydrodynamic transmission.